Generally, a brake system of a vehicle includes a pneumatic booster disposed between a brake pedal and a master cylinder. This type of pneumatic booster boosts a pressing force applied from the brake pedal to an input rod according to a pressure difference between a constant pressure chamber and a variable pressure chamber inside the pneumatic booster. By this operation, the pneumatic booster is configured to transmit a large output from an output rod to the master cylinder side. The constant pressure chamber (i.e., a negative pressure chamber) of the pneumatic booster is connected to an intake manifold of an engine for the vehicle, and the constant pressure chamber is set into a lower pressure state than an atmospheric pressure with the aid of a negative pressure generated in the intake manifold. A biasing member (i.e., a return spring) is provided at the pneumatic booster so as to be able to retort a valve body to the variable pressure chamber (i.e., the input rod) side when a braking operation by the brake pedal is released (PTL 1).